1. Field of the Invention
The invention relates to magnetostatic wave (MSW) filters and, in particular, the invention relates to MSW having a relatively sharp upper cut-off frequency.
2. Description of Related Art
MSW filter elements exhibit sharp attenuation of the signal at the so called lower frequency cut-off and exhibit gradual attenuation at the upper frequency cut-off. Higher selectivity may be achieved when the upper cut-off frequency is sharpened. MSW filter banks and channelizers offer potential of high selectivity over multi-GHz bands with individual passbands in the range of 10-50 MHz, with relatively low loss and extremely small size. The individual passbands have well known shapes which are in general influenced by the MSW propagation physics and geometrical aspects of the filter construction.
The MSW filter 10 illustrated in FIG. 1 consists of a yttrium iron garnet (YIG) film 12 suspended over two current carrying transducers 14 and 16. The YIG film 12 is supported by and epitaxially grown on a gadolinium gallium garnet (GGG) substrate 18. An alumina substrate 20 supports the input and output transducers 14 and 16. A static or DC magnetic field H.sub.DC in combination with the geometry of the MSW 10 sets the passband of the filter 10.
In FIG. 2 a curve 19 of transmission loss (db) V. frequency represents the passband of the MSW filter 10 of FIG. 1. The sharp lower edge 24 is due to the fact that MSWs travelling in the X direction (FIG. 1) have a lower cut-off frequency at which all of the spin moments in the system precess or rotate at the gyromagnetic frequency f.sub.o =.gamma.H where H is the applied field and .gamma. is the gyromagnetic ratio 2.8 MHz/oersted.
The gradual or sloped upper band edge 26 of the passband 19 is determined by several factors, including the fact that when current distribution in the X direction 25 of the transducer 16 is fourier transformed, it determines one factor in the frequency response of the filter 10. The frequency response is also shaped by reflection of MSW from the end of the YIG film. If the YIG film 12 is spaced from the transducers 14-16, the higher sidelobes of the transform fade out, leaving a replica of the fundamental lobe, the slope of which depends on transducer current density. However, the technique does not always produce the desired attenuation. The gradual slope is generic to MSW filter elements.